Main.TrailIndexPage | Electrical Systems | Electrical.Lights | Electrical.Rheostat

Rheostat dimmer switch

All instrument cluster and dashboard lighting is controlled by the rheostat or dimmer switch. This component is part of Electrical System.

Definition

When the Pagoda series of cars were first produced, roads and motorways were generally unlit. A rheostat was inserted in the circuit in order to dim the instrument lights to prevent reflections of the instrument cluster in the windscreen.

Nowadays with lots more ambient light, many owners find their dashboard lights very dim, especially on the speedometer and the tachometer. Usually a very dim light is the result of corrosion or failure of the Rheostat. The Rheostat or Dimmer is a coiled resistor wire. The tab with a brass plate moves along the wire, and varies the level of output voltage, thereby reducing or increasing the brightness of the instrument lights.

Rheostat, or Dimmer.
part # - start year & end year
Electrical System
Dashboard Instruments

Maintenance

If the Rheostat fails, you will have no lights or very dim lights on the dash, or you may only get lights on in one particular position of the dimmer knob.
There are two common remedies. Shorting out the Rheostat is often done, because with the high ambient light reflections in the windscreen are no longer an issue.
If you intend to short out the Rheostat, but if you also drive rallies in darker areas, it may be sensible to wire a resistor in series with the shorted Rheostat (experiment with the values but start with low resistance (5 Ohms) and reasonable wattage 25W) with a switch in parallel with the resistor so that it can be bypassed. This will give you two brightness settings: high (when the resistor is bypassed), and medium (when the resistor is in place of the Rheostat).
Alternatively you can Rebuild the Rheostat
Using some WD40 or electrical contact cleaner spray and applying that to the Rheostat coil may provide you with a temporary solution.
Finally, installing higher wattage lightbulbs may not be the best solution as they will generate extra heat in the confined spaces of the dashboard and instruments. Using modern LED replacement lights is an option.

Rebuilding the Rheostat

All instrument cluster, dashboard lighting is controlled by the rheostat or dimmer switch. If it fails, you will have no lights on the dash. But wait! It can be fixed… The first step would be to remove the instrument cluster from the dashboard, and then remove the rheostat.

Once you have the rheostat out, remove the metal cover by prying the two retaining tabs just enough so the metal cover comes off. Careful, it will now fall into pieces. In your hands will be: the ceramic base; a small rotating brass plate; and two small springs that push the brass plate down onto the coiled wire that serves as the variable resistor.

The two problems most commonly found with the rheostat are:

carbon build-up on the brass plates; and
failure of the coiled wire.

The coiled wire is connected at one end to a brass connector that provides electrical continuity and at the other end it is held in place by some sort of adhesive.

The entire coil sits in a channel within the ceramic base. If the coiled wire is broken – you may be able to install another one by using fuse wire. The rheostat spring wire measures 17-18 ohms of peak resistance. Use some JB Weld to hold new spring in place on both ends. The "spring" is actually the resistor used to vary the impedance and hence the amount of current flowing through the lights. The resistance is varied by shorting the spring with the rotating stator. Maximum intensity is a complete electrical short of the resistor; minimum intensity is when the resistor is partially shorted. By stretching the spring, you essentially short-out most of the resistance!

To address the carbon build up on the brass plates you can use an ink eraser if the build-up is not significant, or you can soak all the pieces in some jewelry cleaner (sulphuric acid and urea) and let it soak until clean.

Reassembly is straightforward although there is one trick. When you put the springs back in place and put the rotating brass part on them you need to hold it down with a small knife blade when you re-attach the other half. Once you've got it on just put the metal cover back on and fold the tabs in and you're done.

Getting access to the Rheostat

Repairing the Rheostat is easy, but getting access to it is not. It is what makes this repair costly...

Remove the under-dash cover on the driver's side.
Reach through the wiring and disconnect the tachometer cable from the tachometer.
Remove the thumb screw that holds the keeper on the tachometer. Pull the keeper loose. Note its orientation.
Pull the tachometer from the dash.
The light sockets are spring loaded. Pull on the little shaft where the wire screws on and the whole socket comes out easily.
Set this unit aside.

The main cluster

Go under the hood and disconnect the small black plastic cover tube to the oil filter fitting. Fish this tube loose so that it can be free to be pulled through the firewall.
Find the other tube from the engine block for measuring the coolant temperature. My tube has enough slack that I didn't take it loose. I just carefully worked it so that it too could easily be pulled through the firewall.
Back under the dash. Remove the thumbscrew and keeper that hold the main cluster.
The steering column gets in the way of pulling the cluster straight out. You have to tip the top out first. WATCH THE TUBING. Mine gave a "pop" when the top edge cleared the dash.
Watching the coolant line, rotate the cluster counterclockwise and lay it on the steering column.
Remove the two screws and the plastic cover from the resistor. Solder a small piece of wire connecting the two posts on the resistor.
Put the cover back on and test.

Testing and grounds

When installed in the dash, the ground for the lights comes through the keepers to the housings. So any testing with the housings not installed in the dash will require a jumper. With the resistor shorted out, you get a full 12 volts to the lights. If you have that then the lack of a ground is the problem. This is true of all the lights.
Installing the modified cluster required some force in my case. Watching the tubes, roll the cluster back up right. Re-tape the rubber gasket in place. Start the bottom of the cluster in first and tip the top to insert. PUSH the top in until it "pops" back in place. I did chip my new paint.
Re-install the keeper and thumbscrew. Now check the lights again. I had to wiggle my keeper until it cut into the paint on the inside of the dash to get a good ground. Also the thumbscrew has to be tight enough to make contact with the housing.
Re-install the Tachometer housing. Check the lights again.
DON'T forget to go under the hood and reconnect the Oil Pressure Line.

Shorting Out the Rheostat

1. Disconnect the Battery!

2. Disconnect the “12 prong plug” on the metal bracket that the “Flasher Can” also hangs on.

3. Remove the cover off the back of the male plug (3 screws on a 230 or snaps-off on 250 and 280's.

4. Remove the two wires that go up to the rheostat. One is grey w/violet stripe the other grey w/blue stripe.

5. Hold each terminal pin with a pair of needle nose pliers, heat with a soldering iron until the wire pulls out. While the solder is still hot tap it against the floor and the excess solder will come out and clear the holes.

6. Make a short (insulated) jumper wire and solder it with the original wires in the terminals.

7. Replace the terminal pins back in the plug where they came out.

8. Reinstall the cover on the back of the plug.

9. Reconnect the “12 prong plug”.

10. Reconnect the Battery and check for operation of your dash lights.

The photos below were taken on the workbench, but the job could easily be done in the car with the steering wheel removed.